1. Field of the Invention
This invention relates to a portable image pick-up apparatus having an image sensing device for sensing the image of a subject and outputting a video signal representing the image of the subject, and a signal amplifier circuit for amplifying the video signal output from the image sensing device, as well as to a method of controlling the operation of this apparatus.
2. Description of the Related Art
A portable image pick-up apparatus such as a digital still camera often is provided with a display unit on the display screen of which the image of a subject is displayed. The user decides the angle of view while observing the image of the subject being displayed on the display screen of the display unit.
Depending upon the type of digital still camera, the user may perform focusing manually. That is, the user focuses the camera manually while observing the image of the subject being displayed on the display screen of the display unit.
When the image of a subject is picked up by photography at a dark location, however, the video signal obtained has a low level. As a consequence, the image of the subject being displayed on the display screen of the display unit becomes too dark, making it difficult to perform focusing manually.
In order to facilitate manual focusing, digital cameras which increase the amount of contour emphasis or open the aperture have been made available. Nevertheless, photography at dark locations still involves difficult manual focusing.
An object of the present invention is to make manual focusing comparatively easy even when photography is performed at dark locations.
According to the present invention, the foregoing object is attained by providing a portable image pick-up apparatus comprising: an image sensing device for sensing the image of a subject and outputting a video signal representing the image of the subject; a focusing setting unit for setting autofocus and manual focus selectively; a signal amplifier circuit for amplifying the video signal output from the image sensing device; an amplification-factor changeover unit (amplification changeover means), responsive to setting of manual focus by the focusing setting unit, for changing over an amplification factor of the signal amplifier circuit from a predetermined initial amplification factor to a first display amplification factor that is greater than the initial amplification factor; a display unit for displaying the image of the subject represented by the video signal amplified by the signal amplifier circuit; a trigger signal generating circuit for generating a trigger signal in response to a record command (e.g., pressing of a shutter-release button) applied thereto; and a recording control unit (recording control means), responsive to the trigger signal generated by the trigger signal generating circuit, for recording the video signal, which is output from the image sensing device, on a recording medium.
The present invention provides also an operation control method suited to the portable image pick-up apparatus described above. Specifically, the present invention provides a method of controlling operation of a portable image pick-up apparatus having an image sensing device for sensing the image of a subject and outputting a video signal representing the image of the subject, and a signal amplifier circuit for amplifying the video signal output from the image sensing device, the method comprising the steps of: providing a focusing setting unit for setting autofocus and manual focus selectively; changing over an amplification factor of the signal amplifier circuit from a predetermined initial amplification factor to a first display amplification factor that is greater than the initial amplification factor in response to setting of manual focus by the focusing setting unit; displaying the image of the subject represented by the video signal amplified by the signal amplifier circuit; and recording the video signal, which is output from the image sensing device, on a recording medium in response to pressing of a shutter-release button.
The present invention is such that when manual focus is set, the amplification factor is changed over from the initial amplification factor to a first display amplification factor higher than the initial amplification factor. The video signal obtained by sensing the image of a subject is amplified by the signal amplifier circuit in accordance with the first display amplification factor. The amplified video signal is then applied to the display unit, whereby the image of the subject is displayed. Since the video signal has been raised to a higher level, the image of the subject displayed on the display unit is brightened.
Thus, a bright subject image is displayed on the display unit even if the subject is photographed at a dark location. Because the image of the subject is easier to see, manual focusing is easier to carry out.
The video signal that is recorded on the recording medium is the video signal output from the image sensing device; it does not have its signal level amplified by the amplifier circuit. Thus a video signal representing a subject image the brightness of which is faithful to the actual place of photography is recorded on the recording medium.
When the video signal is recorded on the recording medium, the amplification factor of the signal amplifier circuit returns to the initial amplification factor. As a result, the brightness of the subject image displayed on the display unit is the same as the brightness of the subject image represented by the video signal recorded on the recording medium. This makes it possible to verify the subject image represented by the video signal actually recorded.
Further, even if the construction of the portable image pick-up apparatus is such that the video signal amplified by the signal amplifier circuit is recorded on the recording medium, a video signal representing the image of a subject the brightness of which conforms to the actual place of photography will be recorded on the recording medium.
It is preferred that the portable image pick-up apparatus further include a photometer unit for metering luminance of a subject, and a subject-luminance discrimination unit for determining whether a luminance value of the subject metered by the photometer unit is less than a predetermined threshold value. In this case, the amplification-factor changeover unit changes over the amplification factor of the signal amplifier circuit from the initial amplification factor to the first display amplification factor in response to a determination by the subject-luminance discrimination unit that the luminance value of the subject is less than the predetermined value and, moreover, to setting of manual focus by the focusing setting unit.
Thus, when the luminance of the subject is less than the predetermined threshold value, the amplification factor of the signal amplifier circuit is changed over from the initial amplification factor to the first display amplification factor. When the image of the subject to be sensed is dark and becomes difficult to see when it is displayed on the display screen of the display unit, the image of the subject is brightened.
The portable image pick-up apparatus may further include a setting unit for setting the amount of brightness adjustment, and a brightness adjustment unit for performing a brightness adjustment in such a manner that the brightness of the image of the subject displayed on the display unit is made greater than initial brightness.
There are cases where the image of the subject displayed on the display screen of the display unit is still dark even when the level of the video signal is raised by the signal amplifier circuit. Since the amount of brightness adjustment can be set, it is possible to brighten the image of the subject displayed on the display screen of the display unit.
The amplification-factor changeover unit can be adapted so as to change over the amplification factor of the signal amplifier circuit from the initial amplification factor to a second display amplification factor larger than the initial amplification factor in response to setting of autofocus by the focusing setting unit.
The brightness adjustment unit is capable performing adjustment in such a manner that the brightness of the image of the subject displayed on the display unit becomes greater than the initial brightness in response to setting of manual focus by the focusing setting unit.